1. Field of the Invention
The present invention relates to a solid-state image sensing device and a solid-state image pickup apparatus using the same, and more particularly to a solid-state image sensor or similar solid-state image sensing device of the type converting incident light representative of a field to an electric signal and dealing with bidimensional image data based on the electric signal. Also, the present invention relates to a solid-state image pickup apparatus of the type catching incident light with the above image sensing device and executing signal processing on the resulting bidimensional image data to record and/or transmit an image produced by the signal processing.
2. Description of the Background Art
Today, a solid-state image sensor, for example, mounted on a digital camera or similar solid-state image pickup apparatus is required to implement both of higher image quality with a greater number of pixels and higher reproducibility of shooting conditions. To enhance reproducibility, providing an image sensing device with a broader dynamic range is under study.
Japanese patent laid-open publication No. 298175/1992, for example, discloses a solid-state image pickup apparatus configured to reproduce an image with a broad dynamic range without causing fixed pattern noise to be involved in a highlight portion. A highlight portion refers to, when bright light, e.g., the quantity of light three to five times as much as one enough to generate a standard signal level, is incident, an extremely light, unreproducible portion is included in the resulting image and viewed simply white or white clip. Also, fixed pattern noise refers to noise appearing as irregularities in a threshold value, which is assigned to the read gate portions of photo-sensitive cells, due to the improvement of a dynamic range implemented by a so-called knee characteristic that compresses data of highlight portions.
More specifically, the image pickup apparatus taught in the above document includes a plurality of sets of photo-sensitive cells which have the optical sensitivity characteristic for photoelectric conversion different from each other. The sets of photosensitive cells form horizontal lines of higher and lower sensitivity cells adjoining each other. Those photosensitive cells are arrayed in bidimensional. With this configuration, the image pickup apparatus reads out signal charges stored in the individual set of photo-sensitive cells and uses, particularly adds, at least one of the resulting two kinds of signal charges in accordance with the quantity of incident light for thereby providing an image with high sensitivity and a broad dynamic range. The signal charges read out from the high-sensitivity photo-sensitive cells are clipped by the output circuit or an external circuit, and then added to the signal of a corresponding pixel, so that fixed pattern noise ascribable to irregularities in saturation is reduced.
The image pickup apparatus provided with the above configuration has a problem that the layout of photosensitive cells is noticeably limited because signal charges stored in the high-sensitivity and low-sensitivity photo-sensitive cells should be read out independently of each other. Moreover, when an extremely light or dark scene is shot, signal charges are read out only from the high-sensitivity or the low-sensitivity photo-sensitive cells, respectively. Stated in another way, only one half of all pixels available with the image pickup apparatus are used for forming an image. The resolution of the resulting image is therefore only one half of the originally expected resolution, thus lowering the image quality.
Some different methods are available for providing photo-sensitive cells with different degrees of sensitivity. A first method provides each photo-sensitive cell with a particular photo-sensitive area, while a second method assigns a particular exposure time to each photo-sensitive cell. Further, a third and a fourth method provide each condenser lens with a particular shape and a particular optical transmissivity, respectively.
Assume that the third or the fourth method is applied to photo-sensitive cells having the same photo-sensitive area for thereby implementing a difference in sensitivity on the basis of optical characteristic. Then, despite that photo-sensitive cells with high sensitivity generate signal charges in dependence upon the quantity of incident light and output them, signal charge levels above a preselected threshold are clipped. Therefore, the photo-sensitive cells with high sensitivity are lower in S/N (Signal-to-Noise) ratio than the photo-sensitive cells with low sensitivity.
Further, it is generally accepted that when a subject to be shot has its luminance distribution narrow, only signal charges read out from the high-sensitivity photo-sensitive cells suffice to output an attractive image. However, the image pickup apparatus stated above shoots a subject completely without considering which of the high-sensitivity and low-sensitivity photo-sensitive cells should be used. Particularly, when the image pickup apparatus produces an image only with the low-sensitivity photo-sensitive cells, the image lacks a sufficient S/N ratio, and is therefore lower in resolution and other factors that determine the image quality.